Semiconductor devices are typically protected against overheating by providing an efficient cooling system for the power module. Power modules can be cooled directly or indirectly, and may have a base plate which can be flat or structured. Conventional base plates are typically made of copper for high heat conductivity. Other lower-cost materials such as AlSiC, aluminum or clad materials can be substituted for copper in the base plate. The base plate can be excluded from the power module to even further reduce cost, but the thermal performance is worse in comparison with modules that use a base plate.
Power modules with and without a base plate are conventionally cooled indirectly by an air or fluid based cooler. The heat generated by the semiconductor devices flows through a ceramic substrate of a DCB-stack (DCB=directed copper bonded), and also through the different solder layers including chip-soldering and system soldering and finally through the base plate. The thermal contact for heat conduction between the base plate and cooler (or between the substrate and cooler if a base plate is not present) is typically realized by thermal grease provided between the base plate or substrate and the cooler. Cooling is less than optimum with such an arrangement because thermal grease has a low heat conductivity of about 1 W/mK.
Directly cooled power modules with a structured base plate more efficiently cool the power devices. The base plate typically has cooling structures such as pins, fins or fin-like structures on the cooling side of the base plate and which are in direct contact with the cooling liquid such as water, or a water glycol mixture so that high heat transfer coefficients are achieved. Different technologies for fabricating the base plate include metal injection molding process (MIM) or forging technology. In both cases, the cooling structures are not directly attached to the cooling side of the base plate. Instead, one or more intermediary materials such as solder, etc. are used to attach the cooling structures to the base plate which degrades thermal performance and increases module cost. Alternatively, the metallized surface of a ceramic substrate can be structured by etching. However, the length of the resulting structures is limited by the thickness of the metallization layer (e.g. about 600 μm for copper). Such structures are an integral part of the substrate metallization and therefore not attached to the substrate metallization, but rather have a continuous construction with the metallization. These cooling structures provide limited thermal capability. A structured DCB can be fabricated with micro deformation technology (MDT) which yields a structure length of about 1 to 2 mm. The resulting structures also have a continuous construction with the substrate metallization and a relatively small cross-sectional width, likewise limiting the thermal capability of the module.